1. Field of the Invention
The present invention relates to a disk drive having a loading/unloading mechanism for loading or unloading a head slider for writing or reading information to or from a recording medium, a loading/unloading apparatus, and a method for controlling the apparatus.
2. Description of Related Art
In a disk drive which rotates a recording medium such as a magnetic disk and raises a head slider mounted on a suspension assembly comprising a head arm of an actuator mechanism from the disk surface by an air bearing produced due to the rotation of the disk, the head slider lands on an unloading area on the disk surface under a non-operation state in which the disk stops its rotation. The unloading area is formed outside of a data area. This type of disk drive is referred to as a contact-start/stop-type disk drive.
Problems encountered in above described contact-start disk drive, include the hazard that the head slider may be attracted by the data area surface or may be moved to the data area due to impact, both of which would damage the disk surface. Therefore, to avoid such hazards and improve the reliability under a non-operation state, a disk drive referred to as a head-loading/unloading-type disk drive was developed. The head-loading/unloading-type disk drive is provided with a head loading/unloading mechanism. The head loading/unloading mechanism includes a head holding mechanism comprising a head-arm suspension assembly and a ramp block which unloads a head slider to an unloaded position without bringing the head slider into contact with the surface of the disk by making the ramp block hold the suspension assembly when the disk drive is not operated. A tab having a protrusion is provided for the suspension assembly and a ramp is formed on the ramp block. The ramp block is set so that the ramp is located nearby the outer track on the disk. The ramp surface is a composite plane having a fist slope, second slope, and tab holding plane. It is also possible to form a horizontal plane (apex plane) between the first slope and the second slope.
To stop the operation of the disk drive, the head loading/unloading mechanism unloads the head slider by rotating the head arm and mounting the tab protrusion of the suspension assembly on the tab holding plane of the ramp. When unloading the head slider, the tab protrusion first contacts the first slope of the ramp, slides on the first and second slopes, and reaches the tab holding plane. Moreover, when the disk drive starts its operation, the head loading/unloading mechanism rotates the head arm to load the head slider to a position above the surface of a rotating disk. When loading the head slider, the tab protrusion slides on the tab holding plane, the second and first slopes and then separates from the first slope.
One limitation of the head loading/unloading mechanism is that it is not generally provided with a special sensor for detecting the position and moving speed of the head slider on the ramp and planes. The speed of the actuator may be controlled by detecting a back electromotive voltage generated at the both ends of a head-moving DC motor (for example the voice coil motor or VCM). The actuator including the VCM is driven by a VCM driving circuit. However, when the actuator moves, a back electromotive voltage is generated at the both ends of the VCM coil. By detecting the back electromotive voltage generated at both ends of the VCM coil and using it as a control object, it is possible to control the speed of the head slider under loading or unloading. To perform stable speed control, it is indispensable to accurately detect a back electromotive voltage serving as speed control information.
However, a disk drive provided with this type of conventional head loading/unloading mechanism has a problem in improving the reliability of the speed control of a head slider under loading or unloading because of the following reasons.
First, when measuring the back electromotive voltage generated at the both ends of the VCM coil with an AD converter and using it as a control object, the current supplied to the VCM coil and the present reading of the AD converter may not show a linear characteristic. That is, a nonlinear area which is not showing a linear characteristic actually expected from the viewpoint of the characteristic of hardware is present. Therefore, to accurately control a head slider under loading or unloading, it is necessary to perform speed control within a linear-operation range. The nonlinear area may fluctuate depending on the operating condition even in the case of an individual apparatus or the same apparatus. Moreover, as described later, because there is a fluctuation in offset values and a fluctuation in VCM coil resistances due to temperature, it is necessary to control speed in a linear operation range by also considering these fluctuation factors.
The speed of a head slider is conventionally controlled without detecting an effective linear operation range of hardware. Therefore, if a control current exceeds a linear operation range and reaches a nonlinear operation range when, for example, the friction of the ramp increases, oscillation or an unexpected control error occurs and speed control may be disabled. As a result the disk may be damaged and user data lost. Moreover, optimum speed control may not be performed because an effective linear operation range of hardware is not detected.
Second, when measuring a back electromotive voltage generated at both ends of the VCM coil with an AD converter and using it as a control object, the value of the AD converter when a control current is zero (that is, offset value) is necessary in order to compute a back electromotive voltage component.
Conventionally, current is actually decreased to zero and the present value of an AD converter is used as an offset value. However, because the current is zero, no force is applied to the actuator at all. Therefore, if a disturbance in the rotational direction is received during the above period, the actuator easily moves while an offset value is measured. Then, a back electromotive voltage is generated because the coil crosses the magnetic field of a VCM, the magnetic field influences the measurement by the AD converter, and thus any accurate offset value cannot be obtained. Moreover, even if voltage fluctuation occurs in addition to the above disturbance, it may influence the measurement of an offset value.
If speed control is started before an accurate offset value is obtained, as control is performed by using the difference from an offset value as the present speed, it is impossible to set a correct target and load a head on a disk at a safe speed. Therefore, the possibility that the disk will be damaged increases and the reliability of data is influenced. As a result, for same reason, greater time is required for unloading.
Lastly, a back-electromotive-voltage detection circuit for detecting the above back electromotive voltage comprises a bridge circuit of a resistance using an operational amplifier. The voltage at both ends of the VCM coil is detected as a back electromotive voltage component by balancing resistance values. However, the VCM coil is subject to temperature changes and the resistance value of the coil greatly changes depending on temperature. For example, when the coil temperature rises, a back electromotive voltage is inaccurately detected due to collapse of the balance of the circuit and a maximum current to be applied to the coil is limited. Because a sufficient amount of current cannot be obtained for the above reason, instability is added to the speed control of a head to a disk. Similarly, then the temperature is low (for example when a power supply is turned on), the resistance value of the coil is lowered, the circuit balance collapses, and a maximum current is limited. This may cause the inability to load the head onto the disk from the ramp.
As described above, in the case of a disk drive provided with a conventional head loading/unloading mechanism, the reliability of speed control of a head slider under loading or unloading is reduced because the risk of disk damage caused by performing speed control in a nonlinear area due to hardware characteristics when detecting a back electromotive voltage. Moreover, offset value fluctuation impairing the speed control state in the effective linear area of the hardware and the change of coil resistance values due to temperature change occur.
It therefore can be seen that there is a need to provide a more accurate apparatus and method for controlling speed in the loading and unloading of a head slider to/from a disk surface.